Field of the Invention
The present invention relates to a process for manufacturing fermented dairy products, in particular stirred milk products.
Description of Related Art
Lactic acid bacteria are extensively used for production of fermented foods, and they greatly contribute to flavor, texture and overall characteristics of these products. An old and well known example is yoghurt which probably originated from the Middle East and which still makes up more than half of the fermented milk production—or approximately 19 million tons in 2008 (source: Euromonitor). Fermented milks as e.g. yoghurts are popular due to the healthy image and pleasant sensory properties.
In many parts of the world an increasing interest in low fat fermented milk products is seen. This poses significant challenges for lactic acid bacteria culture as well as for the production process because it is difficult to produce low fat fermented milk products without reduction of sensory quality.
Yoghurt is produced from milk that has been standardized with respect to fat and protein content, homogenized and heat treated. Hereafter the milk is inoculated with a culture of Streptococcus salivarius thermophilus and Lactobacillus delbrueckii subsp. bulgaricus in large stirred fermentation or culturing tanks and subsequently fermented to a pH of around 4.5. In addition to the traditional yoghurt culture, a probiotic culture, as e.g. Bifidobacterium, can be applied to add extra heath benefits.
In many countries the produced yoghurt product is transported and stored, for instance at storehouses and shops, at ambient temperature, i.e. at temperatures between 20 and 30° C. As a result of this high temperature the lactic acid bacteria in the yoghurt continue to produce acid. This has a negative impact on the taste and texture of the final yoghurt product. A solution to this problem is a starter culture that stops acidifying the yoghurt at a certain pH when stored at ambient temperature. This concept is referred to as Low Post Acidification.
In order to control final acidity, some manufacturers also use specially selected strains of Lactobacillus delbrueckii subsp bulgaricus that do not post acidify and limited quantities of glucose on the fermentation media.
Consequently, there is a need for an alternative cost-efficient process for manufacturing fermented milk that combines high texture preservation and sustainable acidity control during transport and storage.
As used herein the term “milk base” includes whole milk, skim milk, fat-free milk, low fat milk, full fat milk, lactose-free or lactose-reduced milk (produced by hydrolyzing the lactose by lactase enzyme to glucose and galactose, or by other methods such as nanofiltration, electro dialysis, ion exchange chromatography and centrifugation technology), concentrated milk or dry milk.
“Fat-free milk” is non-fat or skim milk product. Low-fat milk is typically defined as milk that contains from about 1% to about 2% fat. Full fat milk often contains 2% fat or more.
The term “milk” is intended to encompass milks from mammals and plant sources or mixtures thereof. Preferably, the milk is from a mammal source. Mammals sources of milk include, but are not limited to cow, sheep, goat, buffalo, camel, llama, mare and deer. In an embodiment, the milk is from a mammal selected from the group consisting of cow, sheep, goat, buffalo, camel, llama, mare and deer, and combinations thereof. Plant sources of milk include, but are not limited to, milk extracted from soy bean, pea, peanut, barley, rice, oat, quinoa, almond, cashew, coconut, hazelnut, hemp, sesame seed and sunflower seed. Soy bean milk is preferred. In addition, the term “milk” refers to not only whole milk, but also skim milk or any liquid component derived thereof.
As used in this specification, the term “fermented dairy product” or “acidified dairy product” is intended to refer to products which are obtained by the multiplication of lactic acid bacteria in a milk base leading to a milk coagulum. The milk preparation used as raw material for the fermentation may be skimmed or non-skimmed milk, optionally concentrated or in the form of powder. Furthermore, this milk preparation may have been subjected to a thermal processing operation which is at least as efficient as pasteurization. The particular characteristics of the various fermented dairy products depend upon various factors, such as the composition of milk base, the incubation temperature, the lactic acid flora and/or non-lactic acid flora. Thus, fermented dairy products manufactured herein include, for instance, various types of regular yoghurt, low fat yoghurt, non fat yoghurt, kefir, dahi, ymer, buttermilk, butterfat, sour cream and sour whipped cream as well as fresh cheeses.
As used in the present specification, the term “yoghurt” refers to products comprising “lactic acid bacteria such as Streptococcus salivarius thermophilus and Lactobacillus delbruekii subsp. bulgaricus, but also, optionally, other microorganisms such as Lactobacillus delbruekii subsp. lactis, Bifidobacterium animalis subsp. lactis, Lactococcus lactis, Lactobacillus acidophilus and Lactobacillus casei, or any microorganism derived therefrom. The lactic acid strains other than Streptococcus salivarius thermophilus and Lactobacillus delbruekii subsp. bulgaricus, are intended to give the finished product various properties, such as the property of promoting the equilibrium of the flora.”
As used herein, the term “yoghurt” encompasses set yoghurt, stirred yoghurt, drinking yoghurt, Petit Suisse, heat treated yoghurt and yoghurt-like products. Preferably, the yoghurt is a stirred yoghurt or a drinking yoghurt. More preferably, the yoghurt is a stirred yoghurt. The term “yoghurt” encompasses, but is not limited to, yoghurt as defined according to French and European regulations, e.g. coagulated dairy products obtained by lactic acid fermentation by means of specific thermophilic lactic acid bacteria only (i.e. Lactobacillus delbruekii subsp. bulgaricus and Streptococcus salivarius thermophilus) which are cultured simultaneously and are found to be live in the final product in an amount of at least 10 million CFU (colony-forming unit)/g. Preferably, the yoghurt is not heat-treated after fermentation. Yoghurts may optionally contain added dairy raw materials (e.g. cream) or other ingredients such as sugar or sweetening agents, one or more flavouring(s), fruit, cereals, or nutritional substances, especially vitamins, minerals and fibers. Such yoghurt advantageously meets the specifications for fermented milks and yoghurts of the AFNOR NF 04-600 standard and/or the codex StanA-IIa-1975 standard. In order to satisfy the AFNOR NF 04-600 standard, the product must not have been heated after fermentation and the dairy raw materials must represent a minimum of 70% (m/m) of the finished product.
In the present context, the terms “fresh cheese”, “unripened cheese”, “curd cheese” and “curd-style cheese” are used interchangeably herein to refer to any kind of cheese such as natural cheese, cheese analogues and processed cheese in which the protein/casein ratio does not exceed that of milk.
The term “starter” or “starter culture” as used herein refers to a culture of one or more food-grade micro-organisms, in particular lactic acid bacteria, which are responsible for the acidification of the milk base. Starter cultures may be fresh (liquid), frozen or freeze-dried. Freeze dried cultures need to be regenerated before use. For the production of a fermented dairy product, the starter is usually added in an amount from 0.01 to 3%, preferably from 0.01 and 0.02% by weight of the total amount of milk base.
As used herein, the term “lactic acid bacteria” (LAB) or “lactic bacteria” refers to food-grade bacteria producing lactic acid as the major metabolic end-product of carbohydrate fermentation. These bacteria are related by their common metabolic and physiological characteristics and are usually Gram positive, low-GC, acid tolerant, non-sporulating, non-respiring, rod-shaped bacilli or cocci. During the fermentation stage, the consumption of lactose by these bacteria causes the formation of lactic acid, reducing the pH and leading to the formation of a protein coagulum. These bacteria are thus responsible for the acidification of milk and for the texture of the dairy product. As used herein, the term “lactic acid bacteria” or “lactic bacteria” encompasses, but is not limited to, bacteria belonging to the genus of Lactobacillus spp., Bifidobacterium spp., Streptococcus spp., Lactococcus spp., such as Lactobacillus delbruekii subsp. bulgaricus, Streptococcus salivarius thermophilus, Lactobacillus lactis, Bifidobacterium animalis, Lactococcus lactis, Lactobacillus casei, Lactobacillus plantarum, Lactobacillus helveticus, Lactobacillus acidophilus and Bifidobacterium breve. 
As used in the present specification, the term “cooling step” or “cooling step for stopping fermentation” means lowering the temperature of the fermented product in order to stop or to dramatically slow down the fermentation process. The cooling step generally lasts less than one minute, preferably about 10-20 seconds. In an embodiment, during the cooling step, the temperature of the fermented product is lowered of at least 10° C., preferably of at least 15° C. and more preferably of at least 20° C. In a preferred embodiment, during the cooling step, the temperature of the fermented product is lowered of at least 25° C. In a particular embodiment, the temperature of the fermented product after the cooling step is in the range of 5° C. to 30° C., preferably in the range of 10° C. to 25° C., more preferably in the range of 10° C. to 20° C., and still more preferably between 10° C. and 15° C.
The term “weakly post-acidifying” or “low post-acidifying” refers to the acidification profile of a bacterium or a bacterial culture useful in the present invention. Post-acidification is the production of lactic acid occurring after the end of the fermentation. This phenomenon is usually controlled by the cooling of the product after fermentation. Indeed, this cooling step stops or slows down the bacterium metabolism and thus reduces the production of lactic acid. In the process of the present invention, this cooling step has been suppressed thanks to the use of bacterium cultures with weakly post-acidifying properties as a starter. These cultures are characterized by a weak production of lactic acid at fermentation temperature after the end of the fermentation step thereby providing a substantially steady pH value.
Weakly post-acidifying bacterium cultures may be selected by the follow-up of the pH of a final fermented milk base by using any method known by the skilled person. As example, a CINAC system (CINetic ACidification) may be used. In this system a pH meter is connected to a computer recorder and pH is continuously recorded as a function of time to obtain sigmoidal curves representing the acidification. During the follow-up of the pH, the milk is maintained at fermentation temperature in a thermoregulated bath. This method is exemplified in the experimental section.
In a particular embodiment, the weakly post-acidifying culture is a culture having a two-phase acidification profile in a milk base comprising 3.6% (w/w) protein, 1.5 (w/w) fat, 4.6% (w/w) carbohydrates and 2% (w/v) RSM powder (referred to as Dutch milk) and/or 2.9% (w/w) protein, 3.6% (w/w) fat, 4.7% (w/w) carbohydrates (referred to as Chinese milk), as determined by the continuous recordation of the pH as a function of time, comprising an initial period lasting 8 to 24 hours of sigmoidal pH decrease down to a pH value of above 4.0, followed by a period of at least 33 days in which the pH value does not fluctuate more than 0.3 unit. Thus, according to the present invention the weakly post-acidifying culture may be selected as having the mentioned two-phase acidification profile in a screening process involving continuously measuring the pH in a milk base comprising 3.6% (w/w) protein, 1.5% (w/w) fat, 4.6% (w/w) carbohydrates and 2% (w/v) RSM powder (referred to as Dutch milk) and 2.9% (w/w) protein, 3.6 (w/w) fat, 4.7% (w/w) carbohydrates (referred to as Chinese milk), inoculated with the weakly post-acidifying culture, and exemplified in the Examples below.
As used in the present specification, the term “exopolysaccharide-producing strain” or “EPS-producing strain” refers to bacteria which are capable of producing polysaccharides, namely exopolysaccharides (EPS), in their culture medium. EPS-producing strains are particularly interesting in dairy product manufacture because they can provide a ropy character and/or a smooth and creamy texture to a fermented dairy product. The produced polysaccharides can be divided into two groups: homopolysaccharides and heteropolysaccharides. Homopolysaccharides consist of the repeated assembly of a single sugar and can be subdivided into four groups, namely α-D-glucans, β-D-glucans, β-D-fructans and others like polygalactan. Heteropolysaccharides consist of the assembly of several different sugars forming a repeating unit most often containing a combination of D-glucose, D-galactose, and L-rhamnose, and, in a few cases, fucose, nononic acid, ribose, acetylated amino sugars and glucuronic acid, as well as non-carbohydrate substituents such as phosphate, acetyl and glycerol.